1. Field of the Invention
This invention relates to a method and apparatus for automatically testing, monitoring and maintaining a motor vehicle starting system and more particularly to a system by which the condition of the vehicle storage battery, as well as the low temperature starting limit, maintenance requirements and service life thereof can be monitored.
2. Description of the Prior Art
Systems for determining one or two of such battery conditions as battery capacity, state of charge, defects in the battery and the charging system, maintenance requirements and timely indication of the need to replace the battery or alternator/regulator, have been developed to facilitate maintenance of a reliable and safe motor vehicle. Manual determination of these parameters is expensive, time consuming and often has to be conducted at service stations. With the introduction of microprocessors, the battery characteristics, engine starting requirements and operational characteristics of a vehicle can be determined automatically, providing optimum life and safety performance for vehicle components.
Automotive batteries provide power for the vehicle's starting, lighting and ignition (SLI) components of the car. Of these, the starter motor requires the most power and is the first component affected by a weakened SLI battery. The necessity for determining when the battery is on the verge of losing its ability to provide the required power is readily apparent. Such a determination is arrived at by a skilled mechanic during testing of the battery and the charging system at a garage, or by means of a monitoring device located in the vehicle. The latter is advantageous in that it is more convenient and provides timely and unbiased advice.
Generally, the automotive battery puts out a few hundred amperes of current for a few seconds of the starting period, the actual amperage being dependent on the battery capacity and its state of charge, and on design and size of the automotive engine and starter motor. The battery is then recharged by the charging system of the vehicle, which consists of an alternator, rectifier, regulator, voltage and current controllers, typically in several minutes.
Several conditions of component status can cause starting failure of a vehicle. These are: Low state of charge of the battery, loss of the battery capacity, corroded terminals, low level of the electrolyte defective charging system and defective starter motor. A battery status monitor should function to sort out these problems automatically. The monitor should diagnose a problem as it develops and warn the user in advance of imminent failure. However, battery monitors heretofore disclosed do not identify each of the aforementioned problems; most of them attempt to determine only the state of charge of the battery.
Battery monitors originally comprised merely an ammeter and/or a voltmeter located on the dashboard of the vehicle. They provided limited information of a type useful only to knowledgeable drivers. To have the battery tested, the car was garaged, the battery was disconnected from the vehicle's electrical system by a skilled mechanic and discharged through a known load. Alternatively, monitoring instruments were employed to derive the battery status parameters using an a.c. signal without discharging the battery through a load. Such monitoring instruments require that the battery be disconnected from the electrical circuitry of the car. Even then they provide information about the state of charge solely.
Conventionally, the state of charge of the battery has been determined by measuring specific gravity of the electrolyte. This method is predicated on the fact that the specific gravity of the sulfuric acid electrolyte decreases when the battery is discharged and increases when the battery is charged. Normally, the specific gravity of the battery electrolyte ranges from about 1.28-1.30 when the battery is fully charged. Such a method is laborious and cannot predict loss of battery capacity in advance. Furthermore, such problems as corroded terminals are not detected by this method.
An alternative method for determining the state of battery charge involves the continuous counting of charge into and out of the battery. This method is subject to accumulation of error due to the current consumed by gassing in the battery and changes in battery capacity arising from corrosion of the electrode plates and shedding of active material. A device of this type, using a current to frequency converter, is described in U.S. Pat. No. 4,678,999 to J. F. Schneider.
A battery's maximum power output is determined by its voltage and internal impedance. Generally the internal impedance of the battery increases with the life of the battery and with a decrease in its state of charge. In U.S. Pat. No. 4,678,998 to Muramatsu, there is described a device based on this principle. According to Muramatsu, the relationship between battery impedance, remaining capacity and remaining service life is predetermined at different frequencies and stored in memory. To determine the battery condition, a computer evaluates the internal impedance at a few frequencies and looks up to the stored table of predetermined values to obtain the remaining service life of the battery. Such a device is subject to the drawback that each battery or battery group has to be subjected to tests before installation to generate the table of predetermined values.
U.S. Pat. No. 4,665,370 to Holland describes a battery monitor wherein a cranking motor furnishes the load for the battery and an electronic apparatus automatically measures the condition of the battery each time the engine is started. The battery monitor compares the unloaded battery voltage with the voltage under load and indicates that the battery is bad if the voltage differential exceeds a predetermined limit. One disadvantage of this device is the inability thereof to distinguish between a low state of battery charge, a fully charged battery having low capacity and additional faulty conditions of the battery such as corroded terminals or low electrolyte level.